chi ao 04.12 - cpaess
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April 12, 2021
© 2021. All Rights Reserved.
Chi Ao1, Chad Galley1, Axel von
Engeln2, Angela Dorsey1, Thomas
Meehan1, Jeff Tien1, Byron Iijima1,
Panagiotis Vergados1, Kuo-Nung Wang1
1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA2 EUMETSAT, Darmstadt, Germany
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Outline
• Mission overview
• GNSS-RO instrument and status
• Near-real-time (NRT) processing
• Initial assessment results
• Summary
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Mission Overview
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Instruments
Mission Objectives• Operational ocean altimetry to provide
continuity of ocean topography measurements beyond Jason-3
• Global sea surface height to an accuracy of < 4 cm every 10 days, for determining ocean circulation, climate change and sea level rise
• NASA, EUMETSAT, ESA and NOAA partnership with CNES providing technical support
• Operational mission as part of a two-satellite EUROPEAN Copernicus/Sentinel program
• Launch Dates: Nov 21, 2020 (S6 Michael Freilich), Nov 2025 (TBC)• Launch Vehicle: SX F9 for S6-MF; TBD for S6-B• Project: Cat II•Risk Class: B for (AMR-C & LRA); C for GNSS-RO• Spacecraft Bus (Airbus: Cryosat Heritage)•Mission life of 5 ½ years (goal of 7 ½ years)• 1336 km Orbit, 66º Inclination
NASA responsible items in BLUE
• Ku/C-Band Radar Altimeter (Next gen Poseidon: Thales)
• DORIS (Precise Orbit Determination System)
• GNSS Receiver (POD System)• Advanced Microwave Radiometer –
Climate Quality (AMR-C)• GNSS-Radio Occultation (GNSS-RO)• Laser Retro-Reflector Array (LRA)
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Flight System Overview
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AMR-C
StarTracker
SolarArray
POD Antennas(on Spine)
X-BandAntenna
S-BandAntenna
GNSS-ROFore Antenna
Poseidon-4
LRADoris
Instruments Measurement DescriptionsR
ange
Poseidon-4: Radar Altimeter
Measures distance between the satellite and the sea surface; includes SAR mode for improved coastal region measurements
Advanced Microwave Radiometer (AMR-C)
Water vapor path delay corrections; includes new high frequency channels for higher resolution in coastal regions (HRMR)
POD
DORIS POD receiving RF signal from Earth-based beacon system.
GNSS-POD POD receiving RF signal from GPS and Galileo satellites
Laser Reflector Array (LRA)
Supports POD reflecting laser from ground-based stations
RO GNSS-RO
(TriG)
Atmospheric profiles based on measurement of bending angles from GPS and GLONASS satellites
POD: Precise Orbit DeterminationRO: Radio OccultationHRMR: High Resolution Microwave RadiometerNASA instrument
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NASA Level 1 RO Requirements
Sentinel-6 Key L1 RequirementsBaseline
The Sentinel-6 project shall measure bending angle and refractivity profiles using radio occultation observations to infer information on atmospheric temperature and humidity for weather forecasting and climate monitoring.
The following baseline requirements are specific instrument performance requirements for NASA-provided instrument development in support of the baseline mission radio occultation science objectives. There are no Threshold requirements associated with the Radio Occultation measurement.
The GNSS-RO instrument shall be designed with a baseline mission lifetime of three years.
The GNSS-RO instrument shall be capable of providing a minimum of 770 profiles per day for the baseline mission.
The maximum Root Sum Square (RSS) measurement uncertainty of the neutral bending angle shall be per Table 4.1 for each specified altitude range.
Requirement # MSL Altitude Range Bending Angle Uncertainty (microrad)
Vertical Resolution (km)
4.3.2.3 10-20 km 30 0.154.3.2.4 20-30 km 3 1.54.3.2.5 30-60 km 2 1.5
Table 4.1: Bending Angle Measurement Uncertainty
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RO Mission Data Products
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Producer Latency Format User Data AccessGTS Archive
JPL NRT (3 h) BUFR L1b, L2 –
EUMETSAT NTC (60 d*) NetCDF – L1b
EUMETSAT/ROM SAF
NTC (60 d*) NetCDF – L2
JPL NTC (60 d*) NetCDF – L1b, L2
JPL NRT (best efforts) NetCDF – L1b, L2 (for 60 d)
NRT: Near-Real-TimeNTC: Non-Time-Critical
L1b: Bending angle profile with impact parameterL2: Refractivity profile with mean-sea-level altitude
* ~ 16 d actual expected
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GNSS-RO Instrument Status
• The GNSS-RO instrument was turned on 1 week after launch.
• Antenna beam-forming was enabled by default and works as expected.
• Instrument power and temperature were within expected and acceptable limits.
• Receiver team has completed the on-orbit operations needed for instrument checkout and Cal/Val, including beamforming checks, tracking altitude adjustments, deep tracking, and L2C POD tracking confirmation.
• Autonomous reboots occur once every ~3 days (slightly less frequent than expected pre-launch), mainly over the SAA and high latitudes.
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The instrument is healthy with software updates being planned to enhance number of occultations and to address some intermittent issues.
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JPL RO Data Processing
Level Format Description0 Instrument Format Raw POD and RO phase data
1A JPL Specified Excess phase with time
1B BUFR (NRT)NetCDF (NRT, NTC)
Time, location, and bending angle vs. impact parameter
2 BUFR (NRT)NetCDF (NRT, NTC)
Time, location, and refractivity over geometric height
DATA ORBITS CALIBRATION RETRIEVAL
Flight Data (GNSS receiver,
s/c attitude)
Level 0
GNSS Ground Network:
GPS & GLONASS
S-6 Orbits & Clocks
(using JPL GDGPS)
GNSS Orbits & Clocks
(using JPL GDGPS)
Nav Data Bits (using JPL GDGPS)
Extract excess phase delay
Level 1a
Derivebending angles,
impact parameter
Level 1b
Final QC
Refractivity[T, P, q]
Level 2
NOAA/GTS
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Radio Occultation Atmospheric Retrieval Software (ROARS)
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NRT Latency Assessment
• Assessment from Jan, 2021:
• NTC Validation Product Requirement: 60 d from last moment of first sensing to availability at NASA/GES DISC archive– Current assessment is within 16 d based on a typical day (significant margin)– Almost all of the latency is waiting for GNSS Final POD products to be generated
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Stage Mean (min) Maximum (min)
Last sensing time of first occ à GNSS-RO L0 creation time 112.1 119.0
GNSS-RO L0 creation time à ROP start at GOC-DN 3.63 8.72
ROP processing time 6.70 7.35
Delivery to NOAA/GTS 6.5* 6.5*
Total latency 129.0 136.2
Margin on 3 h 51.0 (28.3%) 43.8 (24.3%)
Pre-launch best estimate 150.7 168.5
* per latency budget since dataflow not yet active
17 min allocation
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S-6 POD: Initial Assessment
• Position requirement: 10 cm RMS difference per axis• Velocity requirement: 0.1 mm/s RMS difference per axis with a goal of 0.05 mm/s along-track
• Current assessment of NRT solutions based on orbit overlaps of latest common 2 hours• Current assessment of NTC solutions based on orbit overlaps of common 6 hours• Assessment from Jan, 2021:
• Improvements and analyses are ongoing but initial indications are that requirements should be met with margin
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Latency RMS of position differences (cm) in HCL
RMS of velocity differences (mm/s) in HCL
NRT H: 2.17 C: 0.63 L: 2.34 H: 0.022 C: 0.004 L: 0.014
NTC H: 1.38 C: 0.57 L: 1.36 H: 0.021 C: 0.005 L: 0.009
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Instrument Performance: SNR
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• Setting RO SNRs are consistent with COSMIC-2. • Rising RO SNRs lower due to lower-gain antenna.
S-6 has lower SNRs for rising due to smaller fore antenna
Sentinel-6 COSMIC-2
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Instrument Performance: SNR
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Azimuthal distribution of SNR from S-6 setting (aft-looking) occ matches COSMIC-2
(after adjusting for higher orbital altitude of S-6)
0
500
1000
1500
2000
2500
120 140 160 180 200 220 240
C2 FM2Aft RO vs S6 Aft RO GPS Peak L1 SNRv Day 336 2020
C2 S6
Azimuth (deg)
Peak
L1
SNR
(V/V
)
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Number of Occultations & NRT Profiles
770
Requirement is 770 profiles/day• Derived assuming full GPS and
GLONASS constellation.• Lower number is due largely to
several unhealthy GLONASS satellites (~ 10% drop in total number).
• Some rising GLONASS occultations are not being tracked
• Flight s/w update in progress to optimize GLONASS tracking that will increase number of occ.
~ 831 occ/day acquired~ 730 profiles/day retrieved
The average number of occultations tracked daily has been slightly lower than expected.
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requirementmean
NRT Bending Angle Performance: Initial Assessments
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• Initial assessment focuses on high altitudes (easier to characterize noise).
• Assessment below 30 km ongoing.
Mean BA accuracy was found to meet the mission requirement to better than 2 micro-rad above 30 km.
Estimated BA uncertainty at 45 km from one day of occultations
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Bending Angle vs Reanalysis
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Similar mean/stdev for GPS vs GLO Small bias between setting vs rising
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Setting vs Rising
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• S-6 rising occ retrievals have some issues that have been identified.
• A positive bias above 20 km (~ 0.5% at 30 km).
• Worse profile penetration in the lowest ~ 4 km.
• The issues with rising occultations are believed to be due to processing instead of the smaller SNRs of these measurements.
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Tropics (30S-30N)
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• S-6 setting occ retrievals perform similarly to COSMIC-2.
Collocations (300 km, 2 hr)
Bending angle difference between collocated S-6 & C-2 occultations
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Product validation: NWP partners
• NWP partners:– European Centre for Medium-range Weather Forecasts (ECMWF)– U.K. Met Office (UKMO)– Joint Center for Satellite Data Assimilation (JCSDA) – Others: ROM SAF & NOAA STAR
• Roles and responsibilities:– Passive monitoring to compare Sentinel-6 RO observations to background models (results after about 1
month)– Impact of Sentinel-6 RO products on weather forecasts via trial experiments within 2-3 months
• Status:– Provided NRT BUFR products starting March 22, 2021
• Products from first week in Feb, 2021 will be followed by a month of products starting from Feb 26, 2021– NTC products (generated by EUM) are made available as well.
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Product Release Timeline
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NRT L1B & L2 BUFR products ~ L+3 mos NRT L1B & L2 BUFR products ~ L+6 mos NTC L1B & L2 netCDF products ~ L+9 mos
We are here
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Summary
• Sentinel-6 MF is expected to provide ~ 700-900 GNSS-RO profiles each day for at least 3 years (expected longer), with < 3 hr latency needed for NWP data assimilation.
• It will complement COSMIC-2 by providing global coverage with similar performance.• Together with Sentinel-6B (2025 launch), Sentinel-6 will contribute towards long-term availability of
RO measurements for climate monitoring.• Initial evaluation shows good quality POD solutions and RO retrieved profiles, although some issues
with rising occultations have been noted and are under investigations.• RO data products are now available to our NWP partners for independent assessments of RO
products. Public release is currently expected in ~ June 2021.
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